I want to know if a person is maanglik (a person with kuja doṣa), is it a good sign or bad. If bad, what problems one will face in life?
Manglik is a person with Kuja dosha or Mangal dosha or Chevvai dosha. When mars is placed in 1,2,4,7,8 or 12th place from Sukra(Venus), Chandra(Moon) or lagna, this dosha appears in one's horoscope. It isn't any bad sign or bad omen. Its just a different astrological combination that occurs to few people based on their time and date of birth.
This dosha usually comes into discussion during marriage proposals. It is said that a person with this dosha should usually marry another manglik because their partnership would be great and if not there might be problems in their married life. Also sometimes marriage can happen late for these kind of horoscopes but I dont think its any kind of bad sign. That can be because of finding a suitable match with dosha.
Good Things: People with this dosha are tend to possess chivalry, valour, courage, fearlessness, strength, a sense of purpose in life, discipline and a fiery temperament according to some article.
Bad things: Not sure but heard somewhere that depending on Mars position in their horoscope, person with mangal dosha can have different issues like bad temper, less emotional bonding, health issues etc.